Nuclear

Engineering
ELSEVIER Nuclear Engineering and Design 173 (1997) 333-339
and Design

Basic results of the Russian WWER-1000 surveillance program

A.M. Kryukov a,., Y u . A . N i k o l a e v a, T. P l a n m a n b, P . A . P l a t o n o v a
a Russian Research Centre, Kurchatov Institute, 123182 Moscow, Russian Federation
b VTT Manufacturing Technology, P.O. Box 17042, Espoo, FIN-02044, Finland

Abstract

The surveillance test results of the reactor pressure vessels (RPV) of three Russian WWER-1000 type units
designated Units 1, 2 and 3 are given and the embrittlement rates compared to those predicted by the Russian
Regulatory Guide. The surveillance materials properties measured by manufacturers of the RPVs are reviewed. The
chemical compositions indicate low impurity contents (copper and phosphorus) but nickel contents up to 1.9 wt.% in
some welds. The Charpy test results were available for the surveillance base and weld metals and the heat-affected
zone (HAZ) of the three units. Dependence of the radiation behavior of WWER-1000 RPV steels on metallurgical
variables and the damage dose is considered. The trend curves for the steels under investigation are proposed. © 1997
Elsevier Science S.A.

I. Introduction Due to the significance of microstructures and the

contents of impurities, it is important to investi-
The evaluation of radiation embrittlement, ei- gate the embrittlement rate and the development
ther from direct measurements or by using empir- of it with relevant materials corresponding accu-
ical trend curves, is a necessary part o f any rately to the R P V materials, rather than using test
integrity assessment of an RPV. Radiation em- results produced with materials differing from the
brittlement is one of the primary reasons limiting real ones. WWER-1000 R P V steels being dealt
the technical service life of the pressurized water with in this p a p e r have a somewhat special com-
reactors and therefore the issue has been given position in regard to radiation embrittlement, i.e.,
much attention. The radiation embrittlement due low impurity contents but high nickel contents.
to the operation and service life of the plant Embrittlement of these materials has, according
depends on m a n y factors, such as initial material to the published data, been studied much less than
properties, the composition and heat treatments the WWER-440 R P V steel grades which do not
of the R P V steels, irradiation conditions and the contain nickel and are therefore expected to ex-
neutron fluence. The high contents of certain im- hibit a different irradiation response. The most
purities, in most cases phosphorus and/or copper, recent Russian studies showed that the nickel
have typically been primary reasons enhancing the content alone, i.e., with relatively low impurity
development of embrittlement in R P V materials. contents, could enhance radiation embrittlement
(Nikolaeva et al., 1994; Nikolaev et al., 1995,
* Corresponding author. 1996; Nikolaev and Nikolaeva, 1996). The materi-

0029-5493/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved.

PII S0029-5493(97)00112-X
334 A.M. Kryukov et al./Nuclear Engineering and Design 173 (1997) 333-339

Table 1
WWER-1000 surveillance program

N P P code Type of steel Grade Irradiation time Range of fluence (10 TM n Range of flux (10 ~° n cm 2
(h) cm 2, E > 0 . 5 MeV) s l, E > 0 . 5 MeV)

Unit 1 Forging 15Kh2NMFAA 22320 1.51 14.8 1.88 18.4

Weld Sv- 22 320 1.96 8.44 2.43 10.5
10KhGNMAA
Unit 2 Forging 15Kh2NMFAA 14 060 1.33-8.79 2.63 17.4
Weld Sv- 14 060 1.5-6.87 2.96-13.6
10KhGNMAA
Unit 3 Forging 15Kh2NMFA 18240 2.43 3.71 15.7 23.9
30 000 4.23-3.92 22.3 20.6
Weld Sv- 18240 7.9 10.7 12.0-17.3
10KhGNMAA
30000 3.28-18.0 3.04 16.7

als investigated here provided a good basis to • irradiation temperature of surveillance speci-
study the effects of nickel contents since it was mens is 300-310°C, i.e., 10-20°C higher than
possible to analyze test results measured with the irradiation temperature of RPV walls.
steels having various nickel contents and also All these disadvantages cause uncertainties in
some welds having very high contents of nickel. the surveillance data and the latter may result in
unconservative trend curves developed for esti-
mating the radiation response of reactor pressure
2. The WWER-1000 surveillance program details vessel properties.

The types of surveillance R P V materials under

consideration are shown along with the basic 3. Impact test analysis technique
irradiation conditions in Table 1. Table 2 shows
that the contents of the residual impurities known The tendency of steel to undergo brittle fracture
to be most harmful in respect to irradiation em- was estimated using the ductile-to-brittle transi-
brittlement varied little and were at a low level. tion temperature (DBTT) obtained from the im-
On the other hand, nickel contents were high and pact tests of standard Charpy-V specimens
varied in the welds from 1.21 to 1.88 wt.%, which (10 x 10 x 55 mm3). The D B T T s of materials in
seemed to be very significant. both as-received and irradiated conditions were
The following shortcomings in the W W E R - found using a hyperbolic tangent function fit of
1000 surveillance programs can be pointed out: the form:
• the spectral index of surveillance specimens is E(T) = a + b x t a n h ( T - To)/C (la)
different from that of the reactor pressure ves-
sel wall (4.8-10.5 and ~ 5.7, respectively); where E(T) is rupture energy (J); T is the test
• surveillance capsules are located in the position temperature (°C); a, b, c and To are fitting
with a high neutron flux gradient; moreover, parameters with:
disadvantages of the surveillance capsule de- a = l(uppershelf energy + low shelf energy)
sign (location of specimens in positions not in
b = ½(upper shelf energy - low shelf energy)
contact with each other) resulted in a notable
(lb)
radial variation of the neutron flux in the cap-
sules (the difference between maximal and min- Based on W W E R material data base statistics,
imal flux values was 160-170%); the low shelf energy (i.e., a - b) was chosen to be
 A.M. Kryukov et a l . / Nuclear Engineering and Design 173 (1997) 333-339 335

Table 2
Chemical composition of the WWER-1000 surveillance materials

NPP code Type of steel Chemical composition (wt.%)

C Ni P Cu Mn Si Cr Mo V

Unit 1 Forging 0.14 1.13 0.010 0.040 0.41 0.23 1.81 0.53 0.08
Weld 0.07 1.88 0.009 0.028 1.10 0.31 1.72 0.68 0.00
Unit 2 Forging 0.16 1.16 0.007 0.050 0.48 0.24 1.83 0.55 0.08
Weld 0.04 1.76 0.010 0.040 0.98 0.28 1.71 0.66 0.00
Unit 3 Forging 0.17 1.35 0.010 0.120 0.45 0.26 1.74 0.57 0.10
Weld 0.06 1.21 0.014 0.040 0.53 0.25 1.58 0.65 0.01

5 J. The criterion specified by the Russian Guide The common trends of the radiation-induced
(Anon., 1989) was used for determination of the mechanical property change of the WWER-1000
transition temperature, that is, D B T T was to be surveillance materials are analyzed here by ne-
maximal of TT47 and ( T T 7 0 - 30°C), where TT47 glecting the direct dependence of C H E M on
and TT7o are transition temperatures with the chemical compositions, i.e., using the following
reference levels of 47 and 70 J, respectively. equation:
D B T T in unirradiated conditions (TTo) were
ATT = A r × F b (3)
measured for the surveillance materials by both
the manufacturers of the RPVs (i.e., Izhora Plants where A r is the radiation embrittlement coeffi-
and Atommash) and R R C 'K'. The former values cient. It is physically justified to suppose parame-
are to be used for determination of RPV end-of- ter b in Eq. (3) to be positive. Moreover, it can be
life and the latter for determination of the steel found in the literature (for instance, Anon., 1989;
irradiation response. All these values of D B T T Nikolaeva et al., 1994; Nikolaev et al., 1995, 1996;
are listed in Table 3. Nikolaev and Nikolaeva, 1996) that parameter b
for Russian-type RPV materials ranges from 1/3
to 1.
4. Radiation embrittlement of WWER-1000 RPV The trend curve analysis was only performed
materials for surveillance materials that were exposed to
irradiation for at least two fluences. Parameter b
It is usual for Russian-type RPV materials to (Eq. (3)) was found to range from 1/3 to 0.45.
assess the transition temperature shifts due to However, the difference between standard devia-
irradiation using a power function of the fast tions for trend curves with the 'best fitted' b and
neutron fluence of the following type (Anon., b = 1/3 was found to be negligible. The trend
1989; Amaev et al., 1993; Nikolaeva et al., 1994; curves shown in Fig. l(a)-(c) were plotted using
Nikolaev et al., 1995, 1996; Nikolaev and Niko- Eq. (3) with b = 1/3. The trend curves shown in
laeva, 1996): Fig. l(a)-(c) permit a preliminary assessment of
the embrittlement of the RPV materials resulting
ATTF = T T F - TT0 = C H E M x Fb (2)
from irradiation with the designed end-of-life
where b is a constant for which The Russian fluence (--~5 x 1019 n cm -2, E > 0 . 5 MeV). The
Guide (Anon., 1989) specifies a value of 1/3 and D B T T shifts for three different RPV materials
C H E M is a function of chemical variables ('chem- due to irradiation to the end-of-life fluence is
istry factor'). The value of C H E M depends on the expected to be more than 80°C (expected end-of-
steel and is specified to be 230C for the life D B T T shift) (Fig. l(a) and (b)). There is no
1 5 K h 2 N M F A A type base metals and 20°C for base metal among these materials. The highest
their welds (Anon., 1989). D B T T shifts were predicted for the Unit 2 weld
336 A.M. Kryukov et al. / Nuclear Engineering and Design 173 (1997) 333 339

Table 3
DBTT values measured for the WWER-1000 surveillance program

NPP Type of steel Grade DBTT measured by RPV manufacturer DBTT measured by RRC 'KI'

Unit 1 Forging 15Kh2NMFAA - 55 - 82

Weld Sv-10KhGNMAA 0 - 32
HAZ Sv-10KhGNMAA -- - 73
Unit 2 Forging 15Kh2NMFAA - 20 - 40
Weld Sv-10KhGNMAA - 10 -33
HAZ 15Kh2NMFAA -- -41
Unit 3 Forging 15Kh2NMFA - 80 - 67
Weld Sv-10KhGNMAA -50 -20
HAZ 15Kh2NMFAA -- 57

(Fig. l(b); ~ 130°C) a n d the U n i t 1 weld (Fig. T h e e x a m i n a t i o n p e r f o r m e d i n d i c a t e d the sensi-

l(a); ~ 110°C). F o r the o t h e r materials, r a d i a t i o n tivity o f the r a d i a t i o n e m b r i t t l e m e n t o f the
e m b r i t t l e m e n t is expected to be m o d e r a t e . T h e surveillance m a t e r i a l s (welds, base m e t a l s a n d
results o f the C h a r p y tests o f the surveillance H A Z ) to c h e m i c a l c o m p o s i t i o n . T h e s t a n d a r d de-
m a t e r i a l s are s u m m a r i z e d in T a b l e 4. AF(95%) v i a t i o n (~r) was used as a m e a s u r e o f the scatter o f
s t a n d s in T a b l e 4 for the r a d i a t i o n e m b r i t t l e m e n t the e x p e r i m e n t a l d a t a in respect o f the e s t i m a t e d
coefficient d e t e r m i n e d with 95% confidence: t r e n d curve:
A F ( 9 5 % ) = A F ( m e a n ) + 1.96a (4)
a = (ATT~rend . . . . . . ATTob ..... d )2/N (6)
Here, a is the s t a n d a r d d e v i a t i o n c h a r a c t e r i z i n g i=1
the a c c u r a c y o f A F ( m e a n ) e v a l u a t i o n .
T h e regression analysis p e r f o r m e d for all the
available surveillance m a t e r i a l s gave the following
t r e n d curve:
5. The influence of metallurgical variables on the
radiation stability of WWER-1000 type steels ATTaU availabledata = 17.1 x F 1/3 (0" = 17.3°C)
(7)
T o investigate the effect o f the c o m p o s i t i o n o n
the r a d i a t i o n e m b r i t t l e m e n t , it seems r e a s o n a b l e T h e scatter was, however, very high (Eq. (7)).
to use Eq. (2) with the c h e m i s t r y f a c t o r C H E M o f This t r e n d curve s t r o n g l y u n d e r e s t i m a t e s the em-
the form: b r i t t l e m e n t o f s o m e materials. T h e c o r r e l a t i o n
between the o b s e r v e d a n d p r e d i c t e d d a t a seems to
C H E M =f(CNi; CMn; Ccr x Cc; Ccr ;,( C C be low as well. T o clarify the m e c h a n i s m s o f the
X CNi; Ccu; Cp; CpCsi; CNiCsi; ...) (5) r a d i a t i o n d a m a g e for the surveillance materials, it
is r e a s o n a b l e to c o n s i d e r the base metals, H A Z
A l l c o m b i n a t i o n s o f chemical v a r i a b l e s m e n -
a n d weld m e t a l s s e p a r a t e l y as follows:
t i o n e d in Eq. (4) ( a n d s o m e others) were consid-
ered here. T h e chemical c o m p o s i t i o n o f the A T T f°rging = 12.6 x F 1/3 (0-= 11.5°C) (8)
m a t e r i a l s u n d e r investigation (Table 2) m a d e it
ATTHAZ = 16.2 x F 1/3 ( O - = 10.4°C) (9)
i m p o s s i b l e to u n i q u e l y d e t e r m i n e a n y d e p e n d e n c e
o f C H E M o n the c o n t e n t s o f elements o t h e r t h a n A T T weld = 21.5 x F 1/~ ( a = 21.7°C) (10)
nickel.
It s h o u l d be p o i n t e d o u t t h a t all o f the regres- T h e c o r r e l a t i o n was w e a k e s t for the weld m e t a l
sion analyses d o n e s h o w e d the p o w e r o f the m e a n (Eq. (10)). T h e s t a n d a r d d e v i a t i o n s for the base
fluence d e p e n d e n c e to be a b o u t 1/3, i n d e p e n d e n t l y m e t a l a n d H A Z (Eqs. (8) a n d (9)) were f o u n d to
o f the m a t e r i a l t y p e a n d the f u n c t i o n a l f o r m o f be relatively low, which implies a w e a k depen-
the c h e m i s t r y factor. dence o f the e m b r i t t l e m e n t sensitivity o n the basic
 A.M. Kryukov et a l . / N u c l e a r E n g i n e e r i n g a n d D e s i g n 173 (1997) 3 3 3 - 3 3 9 337

alloying elements and residual impurities in their Table 4

ranges o f variation characteristic o f the materials Radiation sensitivity of WWER-1000 surveillance materials
(Table 2). NPP code Material Radiation embrittlement coeffi-
The a c c o u n t o f the nickel content in Eq. (5) cient
slightly decreased the standard deviation for the
predicted radiation response: AF(mean) AF(95%) AF(guide)

Unit 1 Forging 19 27 23
Weld 29 32 20
100 .... , .... , .... , .... , .... , .... HAZ 26 32 Not
• - weld R e g r e s s i o n line: A T F = A F * F I/3
O - forging defined
O - HAZ Unit 2 Forging 16 20 23
50-
Weld 35 37 20
HAZ 18 23 Not
o defined
Guide for forging .
- - LL-JJJf--
Unit 3 Forging 8 13 23
Weld 8 10 20
HAZ 14 17 Not
....-".¢, 5,7..-6
defined
-50 I/0
ATTall a v a i l a b l e data = 12.4 × CNi × F 1/3
.... ; .... 1; . . . . 1; . . . . 2; .... 2; .... 30
Fluence. l0 is n / c m 2 ( E > 0 . 5 M e V )
(a -- 14.7°C) (11)
10O ,._.wel(~ ' " " Re'gre$sio;1 lil!*e: ATF--AF*F ~
0 - forging
where Cyi designates the nickel content (wt.%).
W h e n the materials were considered separately,
the a c c o u n t o f the nickel content in C H E M de-
50- ~ G u i ~ "
creased the standard deviation for the predicted
_ radiation response o f the weld metal but slightly
affected the one for the base metal and H A Z :
/.6 _ ~ - - " _ _ .-. . . . . . . . . -

0- •
A T T f°rging = 10.9 x CNi X F 1/3 (ff = 13.8°C) (12)
o q"-'-'-'-
ATTHAZ= 13.4 x CNi x F 1/3 (or = 12.9°C) (13)

.... ; .... i'0 . . . . i; . . . . 2'0 . . . . 2'~ . . . . 30 A T T weld = 12.8 x CNi X F 1/3 (cr = 15.6°C) (14)
FIuence, I018 n / c m 2 (E>O.5MeV)
Thus, the variation in the experimental data o f
20 .... , .... ) .... i .... ) .... i ....
• - weld Regression line: ATF~AF* F 1/3 the weld trend curve significantly decreased, while
O - forging " ~ .

o, O - HAZ ~ ~ Guidel°~ -". the variation in the base metal and H A Z trend
curve slightly increased. The significance o f nickel
in the radiation embrittlement sensitivity o f the
o(.) -20 ' O. O f . . - ~ :-_C-. . . . . .
surveillance materials under consideration seems
clear by c o m p a r i n g the radiation-induced D B T T
,¢-"'7S ~ o
/ 0
shift values o f the materials with high nickel
60.
/
_ - - - - -
_ - - . . . .
contents ( 4 0 - 7 0 ° C ) and the materials with rela-
tively low nickel contents (10-20°C). The effect o f
_,o [] nickel on the radiation response o f the weld metal
.... ; .... ,; .... ~; .... 2'o .... ~'~ .... ~0 is shown in Fig. 2.
Fluence, lO TM n / c m 2 ( E > 0 . S M e V ) M a n g a n e s e has no significant effect on the radi-
ation sensitivity o f R P V materials according to
Fig. 1. Dependence of the DBTT shift on fast neutron fluence. the available publications. However, correlating
338 A.M. Kryukov et al./Nuclear Engineering and Design 173 (1997) 333 339

C H E M with the manganese content produced a The inclusion of any additional metallurgical
lower standard deviation: variables into the trend curves was not found to
ATTall availabledata : 29.1 x CM, x F 1/3 affect significantly the scatter of the experimental
data around the values predicted.
( a = 11.7°C) (15) The fact that the trend curves of the base metal
and H A Z did not depend on the chemical compo-
The corresponding trend curves for the base
metal, H A Z and weld are as follows: sition can be related to the low variation of the
nickel content in these materials and the relatively
ATT f°rging = 28.2 x C ~ x F 1/3 (or = 11.7°C) low nickel content in comparison with the weld
(16) metals.
ATT HAz = 36.1 x CM~ x F 1/3 (0" = 10.9°C) All the conclusions regarding the radiation sen-
(17) sitivity of WWER-1000 reactor pressure vessel
materials were made on the basis of early surveil-
ATT weld = 24.0 x CMn )< F 1/3 (o = 12.6°C) (18) lance sets characterized by low neutron fluences
Thus, the reduced scatter of Eq. (15) is due to and can be revised on the basis of later surveil-
decreasing scatter of the weld trend curve (Eq. lance sets exposed to higher damage doses and by
18). summarizing the surveillance data of all the oper-
It is worth noting that the variation of the ating WWER-1000 type NPPs.
manganese content in the base metal and H A Z
was negligible (Table 2). Eqs. (16) and (17)
confirm that the radiation response of the materi- 6. Conclusions
als does not depend on the chemical composition
in this range of variation. The reduced scatter of (1) Some disadvantages of the WWER-1000
the trend curves (Eqs. 15 and 18) in comparison surveillance programs exist: (a) the spectral index
with those of Eqs. (7) and (10) can be due to some of the surveillance specimens is different from that
positive correlation between the nickel and man- of the reactor pressure vessel walls; (b) the surveil-
ganese contents in the materials. The decreased lance capsules are located in the position with a
scatter of the weld curve of Eq. (19) m a y result high neutron flux gradient; (c) disadvantages of
from a non-linear dependence of C H E M on the the surveillance capsule design (location of speci-
nickel content as follows: mens in positions not in contact with each other)
result in a notable radial variation of the neutron
ATT weld = 15.0 x CMn )< CNi X F 1/3 (a = 9.3°C)
flux in the capsules; (d) the irradiation tempera-
(19)
ture of the surveillance specimens is 305 _+ 5°C,
i.e., 10-20°C greater than the irradiation temper-
Survei'llanc~ weids: " ' R e g r e s a i ' o n l i n e : ' A T F = A F . F I ~ ature of a WWER-1000 reactor pressure vessel
100 • O - Unit-I " / -
• - Unit-2 Unit-2 - 1.76%Ni~,,.-"~ _ _
wall.
(2) The results of the chemical composition
/
J ~ - ~ J n i t - 1 - 1.88%Ni effect investigation: (a) nickel was found to affect
. . - - " Gu! for wo: .........
the radiation sensitivity of the weld surveillance
" materials; (b) the nickel effect was not observed
? ! / . ~/io . .." . .
..., ..,- " '
for the WWER-1000 base metals and H A Z ,
which can be related to the low nickel content in
Utfit-3 - 1 2 1 N N _.
'- - .o. .... ?_. _o. . . . . . . . . . . . . . . . . the materials as compared with the weld metals;
(c) the low variation and amount of the residual
O
.... i .... J .... . .... i .... , .... impurities (phosphorus, copper, etc.) in the inves-
5 10 15 20 25 30
Fluenc¢, I0 TM n / c m 2 ( E > 0 . 5 M e V ) tigated surveillance materials do not make it pos-
sible to reliably determine any dependence of the
Fig. 2. Effect of nickel on radiation stability of weld metal. radiation response on the residual impurities.
 A.M. Kryukov et al./ Nuclear Engineering and Design 173 (1997) 333-339 339

(3) The prediction for the radiation embrittle- Anon., 1989. Calculation Standards for Strength of Equip-
merit corresponding to the end-of-life fluence may ment and Pipes of Nuclear Power Units, PNAE-G-7-002-
86. Erergoatomizdat, Moscow.
not be reliable for the high nickel materials (the Nikolaev, Yu.A., Nikolaeva, A.V., Kryukov, A.M., Levit,
slope of the damage dose dependence of the radi- V.I., Korolyov, Yu.N., 1995. Radiation embrittlement and
ation embrittlement can rise with nickel content; thermal annealing behaviour of Cr N i - M o reactor pres-
the neutron fluence of the investigated surveil- sure vessel materials. J. Nucl. Mater. 226, 144-155.
lance materials is low). Nikolaev, Yu.A., Korolyov, Yu.N., Kryukov, A.M., Levit,
V.I., Nikolaeva, A.V., Chernobaeva, A.A., Vishkarev,
O.M., Nosov, S.I., 1996. Radiation resistance of reactor
pressure vessel materials alloyed with nickel. Atomnaya
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